The alkali metal salts and disalts of phenol sulfonic acid are widely used as intermediates in the synthesis of phenol sulfonate esters. These esters, which are useful as bleach activators, can be prepared from salts and disalts of phenol sulfonic acid by reaction with acid chlorides or acid anhydrides.
These alkali metal salts and disalts of phenol sulfonic acid are most readily prepared in aqueous solution by neutralization of phenol sulfonic acid with an appropriate amount of an inorganic base, but must be isolated and dried prior to reaction with acid chlorides or anhydrides in order to avoid excessive yield losses due to hydrolysis of the acid chlorides or anhydrides to their corresponding acids.
In conventional methods for preparing anhydrous alkali metal salts or disalts of phenol sulfonic acid, also known as phenol sulfonate salts, the salt or disalt is isolated as a solid from an aqueous solution either by crystallization or by evaporation of the bulk of the water. The isolated salts are not anhydrous and further drying is required to obtain a substantially water free product. The final drying may be accomplished using conventional methods such as oven drying or azeotropic distillation. By way of example: U.S. Pat. No. 4,666,636 teaches drying sodium phenol sulfonate dihydrate crystals by heating in an inert atmosphere at a temperature between about 140.degree. C. and 200.degree. C. to produce anhydrous sodium phenol sulfonate; U.S. Pat. No. 4,704,236 teaches drying sodium phenol sulfonate dihydrate "by heating it when dispersed in a high boiling aliphatic or aromatic hydrocarbon solvent such that water and solvent are co-removed"; and U.S. Pat. No. 4,486,327 teaches drying disodium phenol sulfonate in a vacuum oven at 115.degree.-120.degree. C.
As may be seen from the above discussion, conventional procedures for the preparation of anhydrous phenol sulfonate salts can be tedious and require a considerable amount of solids handling. This is further complicated by the fact that these salts and disalts tend to be hygroscopic. As a result, the anhydrous salts need to be protected from atmospheric moisture while in storage and during handling.
It has been attempted in the prior art to avoid the need for isolation in the drying of alkali metal salts of phenol sulfonic acid by azeotropic removal of water from aqueous sodium phenol sulfonate. However, azeotropic dehydration of an aqueous sodium phenol sulfonate is complicated by agglomeration of the salt during the drying process. As the water is removed from the aqueous solution, the salt forms lumps in the reaction vessel and plates out on, or sticks to, the vessel walls. This agglomeration is undesirable because it makes the salts difficult to work with on a large scale and results in inefficient dehydration.
This problem is recognized in the art and is discussed in European Pat. Application 0 294 073, which states on Page 3, at lines 12-15:
" . . . we have found that the dehydration of sodium phenol sulfonate by azeotropy on a bulk scale poses tremendous difficulties in that the product tends to agglomerate and fuse together to form large lumps. These lumps interfere with the subsequent esterification, impairing not only the quality of the product, but also the yield."